This invention relates to intraocular lenses and, more particularly, to composite intraocular lenses which are foldable so as to be insertable through a small incision of approximately 3 to 4 millimeters (mm).
The natural eye, in humans and most, if not all, animals, contains a lens located internally which focuses images on the retina. Either through disease or other naturally occurring processes, or mutations, the lens may fail to function properly. For instance, the lens, by mutation, may have been eliminated from the eye during its formation at birth. Or the lens may be cloudy at birth, or become cloudy over time. This clouding of the lens is known as a cataract, which inhibits the transmission of visual information through the lens to the retina.
The removal of diseased natural lenses has, prior to the advent of intraocular lenses, required a large incision into the eye at the junction of the cornea and the sclera in order to remove the lens. The healing time in such an operation was substantial and the pain was severe. No lens was inserted in place of the natural lens and eyeglasses or external-type contact lenses were employed to help correct restored vision to the patient.
The advent of intraocular lenses, along with the instruments needed to insert them, was a helpful innovation. Surgical incisions were still relatively large, on the order of 10-14 millimeters (mm), and healing time remained substantial; however, this incision length was required for the cataract removal operation, whereas intraocular lens implants required only a 6 or 7 mm incision for their insertion.
More recently, the cataract extraction operation performed by ultrasonic destruction and suction (phacoemulsification) has been developed and allows the removal of the cataract through an incision as small as 3 mm. However, presently this incision must then by enlarged from 5 to 7 mm for insertion of the intraocular lens implant. Experience has shown that the longer the incision, the longer it takes to heal, the greater the danger of rupture of the incision postoperatively, and the greater the danger that scar tissue, which forms during the healing process, will interfere with the patient's vision. Furthermore, the longer the incision and resultant scar, the greater the danger that post-operative astigmatism will occur due to uncontrolled healing of the eye tissue. It is therefore desirable to develop an intraocular lens implant capable of being inserted through an incision not greater than that which is necessary for cataract extraction, i.e., 3 mm.
On proposal for reducing the length of the incision requires the use of a very soft material for the lens, soft enough to permit the lens to be folded, inserted through the incision, and released. One such material is silicone; however, a lens made from silicone is so extremely soft that, as a result, when the lens and its haptics are permitted to unfold in the eye, the almost jelly-like softness of the lens makes it difficult for the surgeon to properly position the lens in the eye. The jelly-like softness of the silicone lens does not provide sufficient rigidity for the lens after insertion and, as the lens moves about, the optical integrity of the lens may thereafter be affected. In addition, the jelly-like softness of this lens makes it difficult to insert, unfold and position the lens via its haptics in the posterior chamber of the eye. It is the posterior chamber into which most intraocular lenses are placed, as it is behind the iris, like the natural lens it replaces.
Furthermore, there is often a compressive force exerted on the lens by ocular structures following surgery, and these can, and have resulted in deformation of the soft lens and haptics to cause reduced vision in the patent. One of these potentially compressing structures, the lens capsule, surrounds the human lens entirely. It is common practice for the lens implant to be intentionally placed within the capsule following the removal of the cloudy lens or cataract from within the capsule. This is widely accepted to be a preferred method for securing the lens implant as great stability is afforded the implant, and its position within the capsule prevents the implant from contacting more delicate ocular structures. Silicone lenses are, however, not recommended for this type of placement within the capsule as the compressive forces from the capsule can produce lens distortion and reduced vision following surgery.
Another material, or combination of materials, which could be used for a soft, foldable intraocular lens would be a poly(hydroxyethyl methacrylate) or pHEMA or other soft acrylic material, and a rigidizing structure on, or in, the lens, made from a relatively hard plastic such as poly(methyl methacrylate) or PMMA such as disclosed in U.S. Pat. No. 4,718,906. However, the lenses shown in this patent are manufactured by combining these two materials by various methods such as embedding one material in the other, or concurrent extrusion processes, or by solidifying the soft material about the hard material.
Furthermore, U.S. Pat. No. 4,718,906 does not disclose a method of polymerizing the soft pHEMA and hard PMMA materials to a satisfactory manner. These two polymers are dissimilar and an attempt to simply polymerize the PMMA to the pHEMA will result in an intraocular lens having an unstable bond which is easily broken. All of the methods disclosed in U.S. Pat. No. 4,718,906 will provide a two- or multiple-piece lens which is ultimately unsatisfactory for use in the eye.
U.S. Pat. No. 4,764,169 to Grendahl discloses an intraocular lens including a small, hard inner lens optic and a soft, pliable skirt surrounding the lens optic. The lens optic material is a relatively hard material such as PMMA, polysulfone, polycarbonate, or like biocompatible material. The soft, pliable skirt is a silicone, hydrogel or like material. However, the specification fails to disclose how a soft hydrogel material is cast or otherwise polymerized to the central hard PMMA material of the lens optic. Furthermore, Applicant has discovered that while it is fairly easy to cast or polymerize HEMA to form a soft hydrogel around a hard PMMA core and obtain a good said bond therebetween, that the opposite procedure of casting a hard PMMA ring around a soft pHEMA core does not produce a good bond.
U.S. Pat. Nos. 4,093,361 and 4,121,885 to Erickson et al. discloses a composite contact lens and method of manufacture wherein the contact lens has a hard center lens portion and a soft peripheral skirt. The skirt portion being a hydrophilic, water-swellable polymer. An inert, water-soluble substance is added to the monomer feed used to form the skirt portion. After curing, the composite contact lens is hydrated, and the water-soluble substance is leached out, thus eliminating any distortions and/or stresses in the skirt portion. However, the material of the hard center and soft skirt are compatible such that they will properly bond together.
U.S. Pat. Nos. 4,687,485 to Lim et al, and currently assigned to the same assignee as the present invention, further discloses a method of joining together various materials to form a composite intraocular lens wherein the optical portion of the lens is manufactured out of PMMA and the outermost portion of the haptics or legs includes a region of compressible material (i.e., various HEMA copolymers). The compressible regions on the haptics provided a lens which alleviated discomfort and/or trauma to the eye tissues at the point of contact between the patient's eye and the compressible region of the intraocular lens haptic.
It is one primary object of the invention to provide a one-piece, small-incision intraocular lens having a soft, foldable optic portion and a hard, stiff haptic portion.
Another primary object of this invention is to provide a novel polymer composition for an intraocular lens utilizing two dissimilar polymeric materials to form a one-piece intraocular lens.
It is another object of this invention to provide a one-piece intraocular lens having a soft, foldable optic portion and a hard, stiff haptic portion while providing excellent biocompatibility with the patient's eye.
It is an alternate object of the invention to provide a one-piece intraocular lens having a soft, foldable optic portion and a hard, stiff haptic portion wherein the stiff haptic portion has a further soft material at its terminus to protect the eye tissues at the point of contact without sacrificing the lens positioning properties of stiff haptics.
It is another object of this invention to provide a method of making a one-piece intraocular lens utilizing two dissimilar polymeric materials wherein one polymeric material is soft enough to fold and the second polymeric material providing a stiffness sufficient to properly hold the intraocular lens in place within a patient's eye.